1. Field of the invention
The present invention relates to a light beam scanning optical apparatus, and more particularly to a light beam scanning optical apparatus which is used as an image writing head or an image reading head of an image forming apparatus such as an elecrophotographic copying machine, a laser printer and a facsimile.
2. Description of Related Art
Conventionally, as a light beam scanning optical apparatus which is used in a printer and an image reader, a type which is provided with a polygonal mirror is well-known. The polygonal mirror is a polyhedron which has a plurality of reflecting facets at its periphery. The polygonal mirror rotates at a constant velocity driven by a motor, and deflects a light beam which is emitted from a light source with each reflecting facet to scan the light beam at a constant angular velocity. The deflected light beam is then imaged on a light receiving surface through an f.theta. optical system.
Required performances of each reflecting facet are (1) flatness, (2) alignment from a center of the polygonal mirror and (3) perpendicularity. The alignment from the center of the polygonal mirror affects the accuracy of a printing start position. However, this can be amended by using what is called an SOS signal which is generated based on detection of the deflected light beam. The perpendicularity affects the accuracy of a printing pitch of each line. Moreover, this also can be amended by having an optical element with an amending function. Inaccuracy in the flatness causes a jitter, and there has been no optical means to amend this jitter. Thus, flatness is especially required of each reflecting facet of the polygonal mirror. Particularly, in recent years, the polygonal mirror has been produced from resin for a lighter weight, mass production and cost reduction. However, it is very difficult to finish off each reflecting facet flat with high accuracy in a resin molding process, and this causes lowering of productivity, and more specifically, causes more defects and takes a longer molding process time.